Pain in the lower left side of the stomach may be caused by ulcerative colitis, ovarian cysts, varicocele, ureteral stones and other factors. 1. Ulcerative colitis: ulcerative colitis is a chronic inflammatory bowel disease, often occurring in the rectum and the sigmoid colon, the patient may appear in the left lower abdomen or lower abdominal pain, may also involve the whole abdomen, usually accompanied by diarrhea, mucous pus and blood stools and other symptoms. 2. Ovarian cysts: if cysts appear on the left side of the ovary, as the size of the cysts increases, pain in the left lower abdomen, lumbosacral pain, abdominal distension and so on may occur. 3. Varicose veins: male patients who have pain in the left lower abdomen may have varicose veins, which is related to venous valve insufficiency, venous return obstruction and other factors, which will lead to local venous dilatation and tortuosity, resulting in pain in one side of the abdomen, and sometimes radiating to the same side of the groin and perineum. 4. Ureteral stones: ureteral stones are mostly unilateral, if the stone occurs in the left lower ureter, it can lead to pain in the left lower abdomen, and can also be radiated to the same side of the groin, scrotum and other parts of the body, usually colicky pain. In addition, adnexitis, pelvic inflammatory disease, prostatitis and other diseases can also cause pain in the left lower abdomen, the patient can promptly consult a doctor for examination to clarify the cause of abdominal pain.